Method of applying zinc-phosphate conversion crystal coating

ABSTRACT

A modified zinc-phosphate conversion coating method, presented as the Absorbed Solution Layer Phosphating (ASLP) coating method. This modified zinc-phosphate conversion crystal coating (ZPCCC) method is carried out in a reactor, containing metal substrate, a quantity of phosphate solution and a quantity of inert, solid filler particles, the quantity of phosphate solution being dependent on the solid filler particle&#39;s solution absorption ability. The ASLP process is carried out in an interface layer between the substrate and the solid filler particles, with the reactor being constructed to provide movement of treated parts and filler particles relative to each other, continually exposing surfaces of the substrate to fresh phosphate solution.

FIELD OF THE INVENTION

The present invention relates to a method of applying phosphateconversion coating, and specifically zinc-phosphate conversion crystalcoating (ZPCCC), on steel, cast iron, various metals and alloys, and ondifferent types of Zinc coatings on metals.

Phosphate conversion coating, and specifically zinc-phosphate conversioncrystal coating, is used for improving paint coatings, rubber coatings,organic, and inorganic coatings to metal surfaces. In addition,zinc-phosphate coating is applied to increase corrosion protection formetals, where the coating works as a carrier for the specific oil or waxfilm that is applied over it.

BACKGROUND OF THE INVENTION

An excellent overview of the Zinc Phosphating processes is presented inthe article by John Donofrio (Metal Finishing, v. 98, N 6, 2000, pp57-73). For the reader's convenience, a portion of this article ispresented here:

-   -   “Zinc phosphate is a crystalline conversion coating that is        formed on a metal substrate, utilizing the chemical reaction        between metal ions that have been dissolved in mineral acids and        then diluted with water to form the process solution. The        mineral acids that are normally used to dissolve the metal ions        are nitric acid and phosphoric acid. Metals such as Zinc, Nickel        and Manganese are dissolved depending on the process necessary.        Accelerators may be added to phosphating processes to increase        reaction speed, modify hydrogen elimination and control sludge        formation.    -   Three primary reactions take place:    -   The first reaction that occurs when the zinc phosphate solution        comes into contact with the metal surface is the pickling        reaction, in which some metal is dissolved from the surface. In        this reaction a chemical cleaning of the surface takes place.        This cleaning affects the adhesion of the coating to the base        metal. The free acid of the solution close to the metal surface        is consumed because of the dissolution of the metal surface.        Metal ions are transferred into the process solution. The type        of metal depends on the type of substrate mix being treated.    -   The second reaction is the coating reaction. Due to free acid        consumption in the liquid-metal interface, pH rises and the        metal cations can't stay soluble in the solution. They react        with the phosphate in the solution and deposit on the metal        surface as crystalline Zinc Phosphate.    -   The third reaction is the sludge formation reaction. The metal        ion (Fe++) that is dissolved from the pickling reaction is        oxidized using the accelerator and will precipitate out as        sludge. The sludge, created in the process, is normally filtered        out from the solution utilizing some sort of filter media or        equipment.    -   Special prerinses, applied to the metal surface prior to        phosphating, provide a considerable increase in the number of        nuclei for phosphate crystallization. This is termed activation        of the phosphate coating formation.    -   Several different processes can be utilized with Zinc        Phosphating. Straight spray installations are popular, as well        as straight dip operations. Because the spray utilizes the        kinetic energy of the spray pressure, concentrations and        treatment times can be kept lower in spray operations compared        to dip. Heavy Zinc Phosphating processes are applied in dip        operations, either in bulk processes utilizing tumblers, or in a        specific rack design.”

In general, the ZPCCC process proceeds as follow:

-   -   1. The parts, or substrates, to be coated, are immersed in a        solution that contains soluble zinc-phosphates (or soluble        phosphates of iron, nickel, or manganese).    -   2. The solution reacts with a base metal, and as result an        insoluble layer of phosphate crystals is created on the part's        surface. The rate of reaction of this process, and the        properties of the created layer depend on: the concentrations of        the free and bonded Phosphoric Acid; the concentration of the        metal ions; the temperature and the acidity of the solution.

The coating application processes are similar for the various types ofphosphate solutions. Relatively small substrates are loaded into specialimmersion baskets, or rotated drums, and immersed in the phosphatesolution baths. They are immersed for a set dwelling time, and thentransferred to rinsing baths. (Several baths, containing differentsolutions are sometimes required.) The coated parts/substrates are thendried. Large substrates may often be immersed as they are.

Another popular phosphate coating method is spraying the parts, with aspray solution taken from the phosphate solution tank, to coat them.

There are a number of serious technical problems associated with theaforementioned coating application processes. The attempts to solvethese problems, to date, involve: the addition of complex equipment; theaddition of manpower to operate and maintain the complex equipment; andemission control of toxic waste byproducts.

The coating process problems include the following:

-   -   Applying uniform ZPCCC thickness on all areas of the substrate.    -   Maintaining long-term bath solution reactivity.    -   Controlling toxic material emissions.    -   Removing precipitates from the coating immersion bath.    -   Increasing the process cost, due to the addition of expensive        process control components.    -   Maintaining phosphate coating crystal size stability

Applying uniform ZPCCC thickness on all areas of the substrates requirestransferring fresh solution to all surfaces of the part. To date,movable or rotated immersion baskets are used. Alternatively, phosphatesolution spraying is used.

Maintaining long-term bath solution reactivity is difficult. Since allthe base metals react, to varying degrees, with the phosphate solution,they reduce the ZPCCC reaction rate. Some of the base metals, such asAluminum, significantly reduce the ZPCCC thickness. To minimize thesenegative effects often requires the addition of toxic materials, such asFluorine, whose products must later be dealt with. Solution temperatureand acidity control becomes critical.

In addition, the concentration of phosphate is diminished during thereaction with the substrate metal, and consequently, the solutionreactivity and its acidity are reduced. Insoluble phosphates aregenerated in the solution, as well as on the substrate surface.Precipitation removal from the bath poses an additional problem.

To maintain phosphating bath reactive stability requires the addition ofexpensive process control components and computerizing in order tomeasure and control a large number of bath parameters, as discussed inU.S. Pat. No. 5,117,370 to DeCello et al (1992). Phosphating baths aregenerally cleaned every 4-5 weeks, and their coating material totallyexchanged every 9-10 weeks [Surface Engineering, ASTM Handbook, V. 5, p.386].

The process problems, discussed above, are less acute when dealing withthin phosphate coatings, 2-4 g/m2, used as a base for painting oroiling.

The process problems, discussed above, are acute when dealing withcorrosion resistance phosphate coatings that require 10 g/m2 and more.In that case, the coating process parameters, such as phosphateconcentration, acidity, temperature, and activator concentration, mustbe carefully controlled, in order not to reduce solution stability, andas a result solution reactivity.

The phosphate coating rate and phosphate crystals size depend, both, onsolution composition and the surface condition of the substrate. Forexample, grinding or polishing the substrate surface results in smallphosphate coating crystals. Small size phosphate coating provides bettercorrosion resistance and paint adhesion.

Maintaining phosphate coating crystal size stability is an additionalphosphate coating process problem. Special activators, such as TitaniumPhosphate, are used to help create phosphate coating with specificcrystals size. The activator stability and its lifetime in the phosphatesolution depend on several parameters, including: acidity, saltconcentrations, temperature, and surface roughness.

The ZPCCC Mechanism

The ZPCCC mechanism is described in the article by John Donofrio (seeabove). The main principle of this process is that the zinc-phosphatesolution reactions take place in a thin layer above the metal surface.The solution reacts with the metal. The acidity in this thin layerdiminishes. The solution is shifted from its equilibrium state, andconsequently, an insoluble crystal layer is formed on the metal surface.

The bath solution is constantly agitated. As a result, the solution,situated in the thin layer above the metal surface, is constantlyrenewed. Solution that has undergone reaction is mixed in with the bulkbath solution. This solution transfer process decreases the coatingformation rate. It also disturbs the equilibrium of the bulk bathsolution, resulting in the precipitation of insoluble phosphates in thebath solution, and its contamination.

Thick ZPCCC layer creation requires increasing the temperature andadding special activators, which in turn increase the bath solutioncontamination rate.

U.S. Pat. No. 5,399,208 to Sobata et al (1995), suggests performing theZPCCC layer formation in an additional separate bath reactor. Therequired components are added to this separate reactor. After a setperiod of process time, the used solution is removed, cleaned fromcontaminations, and then transferred to the main bath.

This method, utilizing an additional separate bath reactor, increasesthe bath solution lifetime and the ZPCCC process stability. However, thereactor volume must be large enough to maintain a stable process. Thereal, bulk density of steel items is usually 1-4 kg/L. Since the steeldensity is 7.8 kg/L, the unfilled volume of the bulk volume is, on theaverage, 0.7 L/kg. To provide ZPCCC process uniformity, all this volumeshould be filled with solution. Therefore, for a 200 kg batch ofsubstrate, the minimal reactor volume should be 140 L, and in realindustrial situations a volume of 500 L. is reasonable. Therefore, thevolume of solution, required for the process, is a hundred times morethan the volume of solution, actually required for ZPCCC formation. Forexample:

-   Relative surface of treated parts is 0.1 m²/kg-   Batch weight is 200 kg.-   ZPCCC thickness is 4 g/m²-   Total weight of created phosphate salts is 80 g.

Usually the phosphate salt concentration in bath solution is 100 g/L.Therefore, 500 L of solution contain 50,000 g phosphate salts, which is600 times more than was needed for the ZPCCC formation.

Using too much volume of solution results in increased cost due to lossof unusable chemicals and increased ZPCCC bath solution aging.

Therefore, it would be desirable to provide a method of applyingzinc-phosphate conversion crystal coating that possesses the advantagesof the standard ZPCCC process, but utilizes a minimum of phosphatingsolution.

SUMMARY OF THE INVENTION

The above-mentioned process drawbacks are eliminated using a modifiedZPCCC technology development.

The preferred embodiment of the present invention deals with corrosionresistance phosphate coatings that require 10 g/m2 and more.

In accordance with a preferred embodiment of the present invention, andto achieve the goal of providing a uniform coating thickness, whileutilizing a minimum of phosphating solution, there is provided a processreactor volume containing a quantity of relatively small, chemical inertfiller particles. The shape and size of these filler particles may bevaried according to the application. The particle size ranges between 1to 20 mm (usually 2-5 mm). The quantity of phosphate solution, requiredfor the modified ZPCCC process, depends on the filler's solutionabsorption ability.

According to the present invention, the modified ZPCCC process isperformed, using either continuous or periodic movement of treated partsand filler particles, continually exposing substrate surfaces to freshsolution. This method is herein described as Absorbed Solution LayerPhosphating (ASLP).

For the example, described previously, the required filler volume shouldbe 140 L., and the filler particles are present as spheres having a 3 mmdiameter. Experiments show that the phosphate solution volume, absorbedon this quantity of filler is approximately 3-5 L. This volume ofphosphate liquid contains enough phosphate salts for providing therequired thickness of ZPCCC. The ZPCCC reaction is carried out in asmall volume of solution, in an interface layer between the treatedsubstrate and the filler particle. The solution reacts rapidly not onlyon the border between the solution and the substrate, but also in allthe interface layer volume. During the period of time when there ismovement of treated parts relative to filler particles, fresh phosphatesolution is introduced to the interface layer, enabling rapid coatinglayer creation and uniform coating thickness.

As a result, a thick, uniform ZPCCC layer may be created without havingto add any special activators.

Another positive result of the movement of treated parts and fillerparticles is uniformity of the ZPCCC crystals' size. This effect resultsfrom the nucleating of many small, already deposited, phosphatecrystals.

ASLP is carried out in a reactor, which provides either continuous orperiodic movement of treated parts and filler particles. The simplesttypes of reactors use a drum rotated at a speed of 0.2-1 r.p.m., or avibrating machine. Other mixing options involve using the magneticproperties of treated substrates being passed through the filler.

Units for reactor loading and reloading, and for phosphate solutionaddition and removal/exchange are generally required.

Other features and advantages of the invention will become apparent fromthe description and experimental data contained herein below.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention, reference is made to thefollowing drawing, in which like numerals designate correspondingelements or sections throughout, and in which:

FIG. 1 shows the ASLP batch process diagram

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, two possible ASLP batched process options aredescribed below:

-   -   Option 1: Heated phosphating solution is provided from tank 1 to        the ASLP reactor 2, which is already filled with the required        amount of filler particles, during the time required for filler        heating and rinsing. The solution, used for rinsing, is removed        from the ASLP reactor 2 to the cleaning system 4, by gravitation        or by pumping. Cleaning is performed using a filter,        precipitator, or hydro-cyclone. The treated solution is returned        to tank 1, as required.        -   Before phosphating, valve A is turned off and all the            solution from the reactor is removed, except for the            solution absorbed on the filler particle surfaces. A batch            of the substrate, to be treated, is loaded and treated for            the period of time required to attain the required thickness            of ZPCCC. During the ASLP process, periodic or continuous            movement of the treated parts and filler particles is            carried out. After phosphating, the coated substrate may be            rinsed in the reactor, or transferred for required            finishing, for example, rinsing, neutralizing etc., to other            equipment. Finally, the coated substrate undergoes a drying            operation.    -   Option 2: Tank 1 contains heated washing liquid for substrate        washing, before and after phosphating. After the washing liquid        is removed from reactor 2 to the tank, as in the previous        process option, valve A is turned off, and a small quantity of        phosphating solution concentrate is added to the reactor 2. This        concentrate is mixed with a washing solution, which was absorbed        on the filler particles' surface. The quantity of phosphating        solution concentrate is calculated, so that the final        composition of solution absorbed on the filler particles is that        required for the phosphating process. The batch of substrate, to        be treated, is loaded into the reactor and the process continues        as in the previous process option.

EXPERIMENTAL DATA

For a better understanding of the invention, reference is made to thefollowing experimental data:

Phosphating Solution Preparation

Concentrate of phosphating solution (CPS):

-   1. To prepare 1 liter of CPS the following amounts of each component    is used:    -   ZnO—130 g    -   Water—270 g    -   H₃PO₄—510 g    -   NaOH—42 g.-   2. The density of the CPS liquid was increased to 1.300 g/cm³ by    adding water.-   3. Phosphating solutions were prepared by mixing CPS with water or    washing solution-   4. Washing solution was prepared by mixing CPS with water to obtain    a density of 1.040 g/cm³; the pH of this solution was 2.80.

Chemicals Used in Phosphating Solutions

-   ZnO: pigment grade, producer—“Numinor Ltd.”-   H₃PO₄: technical grade, density 1.65 g/cm³, supplier—“Chemilab Ltd.”-   NaOH: technical grade, producer—“Glüsan”, Turkey, supplier—“Chemilab    Ltd.”-   Water: deionized.

Control of Solutions and Phosphating Process:

-   1. Density of phosphating liquid was measured by hydrometer.-   2. Density of CPS was measured by weighing.-   3. Acidity of phosphating liquid was measured by pH-meter.-   4. Crystal size was measured by microscope (average of ten    measurements).

Equipment for Controling Phosphating Process:

-   -   pH-meter model “CyberScan 500 pH”, producer “EUTECH        Instruments”, Singapore, supplier—“M.R.C. Ltd.”    -   Hydrometer with scale in limits 1.000-1.200 g/cm³,        supplier—“Bein Z. M. General Laboratory Equipment”    -   Analytical balance GF-200, producer “A&D Co., Ltd.”, Japan    -   Metallographic microscope model OPTIPHOT-1005, producer “Nikon”,        Japan, supplier “Prisma Ltd.”        Process Equipment used in the Experiments        The experiments were carried out using the following equipment:    -   Plastic drum with volume of 1 liter and diameter 70 mm.    -   Industrial vibrator model CV 250 SG, producer PMG, Japan.

Process Parameters in the Experiments

-   -   1. The method of phosphating was based on the known process of        phosphating in a bath, simulated in laboratory plastic beaker,        having a volume of 0.5 liter.    -   2. In all experiments, the phosphating temperature was 35° C.        and was maintain by thermostat with accuracy +/−2° C.    -   3. Duration of phosphating process (except for specific cases)        was 10 minutes.    -   4. After the phosphating process, the control samples were        washed in running water and dried using paper towels.    -   5. Initial concentrate of phosphating solution (CPS) was        constant.    -   6. Plates, each with an area of 17 cm², produced from steel AISI        220, were used as control samples.    -   7. Before phosphating, the plates were diffusion saturated in        Zinc powder at 410° C. during 1 hour.    -   8. The thickness of Zn—Fe layer that was received by the        above-mentioned method was approximately 40+/−5 micron.    -   9. The phosphate coatings that were obtained in different        experiments on these samples were described using the following        parameters:        -   thickness of phosphate coating, in g/m²        -   crystals size of phosphate coating, in microns        -   corrosion resistance, determined by SST data (in hours from            time that white corrosion began). The results were averaged            no less than by three samples.    -   10. The phosphating solution was prepared via mixing one part of        CPS and three parts of water.    -   11. The density of the phosphating solution was 1.090 g/cm³, pH        was 2.48.    -   12. Part of the solution was used for simulating the phosphating        process in the bath and another part—for the selection of ASLP        process mode.

Experiment Descriptions

The results of experiments are tabulated in Table 1.

Experiment #1:

-   1. 300 ml of solution, prepared as described above, was added to a    plastic beaker, having a volume of 0.5 liter.-   2. The quantity of metal for phosphating was 100 g.

Experiment #2:

-   1. 700 g of porcelain chips, having triple-edged prism shape, with    sides 3*3*4 mm and height 4 mm, was added to a plastic drum.-   2. 300 ml. of solution was added to the drum, covered with a lid,    and was rotated at a rotation speed of 2 rpm, in a horizontal    position during 5 minutes.-   3. The lid was then removed and solution fully removed, except the    solution absorbed on the filler particle surfaces.-   4. The results of weighting show that in the drum, about 28 ml of    phosphating solution was absorbed by chips and drum' sides.-   5. 300 g of metal for phosphating, including three control samples,    were added to the drum.-   6. The drum was closed and revolved with rate 0.3 rpm during 10    minutes.    Experiment #3: Repeated Experiment #2 with the difference being that    the rate of revolution during phosphating process was 2 rpm.    Experiment #4: Repeated Experiment #2 with the difference being that    the rate of revolution during the phosphating process was 5 rpm.    Experiment #5: Repeated Experiment #2 with the difference being that    the rate of revolution during the phosphating process was 1 rpm.    Experiment #6: Repeated Experiment #1 with the difference being that    the density of phosphating solution was 1.0150 g/cm³, pH was 2.35.    The solution was prepared by mixing one part of CPS and one part of    water.    Experiment #7: Repeated Experiment #5 with the difference being that    used the solution prepared for experiment #6.    Experiment #8: Repeated Experiment #5 with the difference being that    1 kg of corundum powder, with average grain size 850 micron, was    added to the drum. The solution quantity absorbed by corundum powder    was 240 ml.    Experiment #9: Repeated Experiment #5 with the difference being that    7 ml of CPS was introduced into the chips before and 21 ml of water    after.

Experiment #10:

-   1. The vibrator was filled with 150 kg of chips (the same size used    for Experiments ##2-9).-   2. Phosphating solution was added to the vibrator and 50 kg of    metal, including three, control samples, were added. The frequency    of the electrical current supplying the vibrator motor was used as    an indicator of the rate of moving chips in the vibrator.-   3. Preliminary experiments showed that the thickest phosphating    layer was formed at a frequency of 20-25 Hz. At lower frequencies    the chips didn't move. At higher frequencies, a decrease of coating    thickness was observed. As the frequency was increased, it was    observed that the coating thickness decreased even further.    Consequently, a frequency of 22 Hz was used.-   4. The time of phosphating was 10 minutes.-   5. The phosphated articles were un-loaded at a frequency of 30 Hz,    during 5 minutes.-   6. At process end, the solution was emptied.-   7. The chips had absorbed 4.5 liter of solution.    Experiment #11: Repeated Experiment #10 with the difference being    that 0.8 liter of CPS and 3.7 liter of water was added to the    vibrator.

Experiment #12:

-   1. To create a closed-cycle system, 600 liters of washing solution    was prepared.-   2. The vibrator containing 150 kg of chips and 50 kg of metal was    washed by this solution during 10 minutes.-   3. After this, the tap providing the washing solution was closed.-   4. The washing solution was pumped into the bath with a device for    sludge settling.-   5. 0.8 liter of CPS was added to the vibrator, and was phosphated    during 10 minutes at a frequency of 22 Hz,-   6. The metal was washed by this washing solution during 2 minutes    and the metal was unloaded from the vibrator and was loaded into the    dryer.    Experiment #13: Repeated Experiment #12 with the difference being    that the phosphating time was 15 minutes.    Experiment #14: Repeated Experiment #12 with the difference being    that the phosphating time was 7 minutes.    Experiment #15: Repeated Experiment #12 with the difference being    that 1.5-liter of CPS was added to the vibrator.    Experiment #16: Repeated Experiment #12 with the difference being    that 2.5-liter of CPS Was added to the vibrator.    Experiment #17: Repeated Experiment #12 with the difference being    that 0.6-liter of CPS was added to the vibrator.    Experiment #18: Repeated Experiment #12 with the difference being    that 0.3-liter of CPS was added to the vibrator.    Experiment #19: Repeated Experiment #12 with the difference being    that a solution, diluted to a density of 1.030 g/cm³, was used    during washing.    Experiment #20: Repeated Experiment #12 with the difference being    that the vibrator was filled with chips of cylindrical form with    diameter 8 mm and length 6 mm.

TABLE 1 Characteristics of ZPCCC obtained in experiments PhosphatingStability Average size layer thickness, in SST, of crystals, # g/cm²hours microns Notes 1 9 144 5 2 6 72 2 The surface color is not uniform.There are areas without a phosphating layer 3 10 192 1 4 2 48 <1 Wideareas of un-coated surface 5 12 240 1 6 10 168 8 7 13 288 1 8 11 264 <19 12 240 1 10 13 312 <1 11 12 264 <1 12 13 312 <1 13 15 336 1 14 8 192<1 15 14 312 1 16 15 312 2 17 12 264 <1 18 8 144 <1 19 13 312 <1 20 13312 <1

As follows from data in the Table 1, the ASLP process performsconsistently in a wide range of varying phosphating solutioncompositions (Experiments #7, 9, 10, 11, 12, 16, 17, 19), varyingphosphating time (Experiments #10, 13, 14), varying filling grains size(Experiments #2, 8, 10, 20), varying speeds of substrate and fillermovement (Experiments #2, 3, 5, 10). At optimal ASLP conditions, theprocess, without adding activators, obtains a very good thickness ofphosphate layer (up to 15 g/cm²), small sizes of crystals (1 micron andless) and high corrosion stability (336 hours in SST).

Having described the invention with regard to certain specificembodiments and examples, it is to be understood that the description isnot meant as a limitation, since further modifications may now suggestthemselves to those skilled in the art, and it is intended to cover suchmodifications as fall within the scope of the appended claims.

1. A method for producing and applying a Zinc-Phosphate ConversionCrystal Coating, providing a uniform coating thickness, while utilizinga minimum of phosphating solution, said method comprising: treatingsurfaces of metal substrates with a zinc phosphate solution, causingdeposition of a crystalline zinc phosphate coating on said surfaces,said treating being performed within a reactor volume containing aquantity of relatively small, chemically inert filler particles; andagitating said filler particles to insure maximization of contactbetween said filler particles and said surfaces, said filler particlesproviding a carrier for said phosphating solution, and enablingminimization of a quantity of said solution required to produce saidcoating.
 2. The method of claim 1 wherein said required quantity of saidphosphate solution is related to a solution absorption capacityparameter of said filler particles.
 3. The method according to claim 1,wherein the size of said filler particles may range between 1-40 mm, andis usually 2-10 mm.
 4. The method of claim 3, wherein the shape of saidfiller is regular.
 5. The method of claim 3, wherein the shape of saidfiller is irregular.
 6. The method of claim 3, wherein a mixture ofregular and irregular shapes of filler is used.
 7. The method accordingto claim 1, wherein said reactor provides continuous movement of saidsubstrate and said filler particles relative to each other within saidreactor volume.
 8. The method according to claim 1, wherein said reactorprovides periodic movement of said substrate and said filler particlesrelative to each other within said reactor volume.
 9. The methodaccording to claim 7, wherein said movement is performed at a speed,which does not deplete the coating layer.
 10. The method according toclaim 1, wherein said required quantity of said phosphate solutioninsures wetting of all surfaces of said filler particles.